1. Field of the Invention
The present invention relates to a sampling frequency conversion filter and, more particularly, to a sampling frequency conversion filter for permitting a digital signal obtained from a CCD imaging device having, for example, 760 pixels a line and a digital signal obtained form a CCD imaging device having, for example, 510 pixels a line to be processed as digital signals of the same sampling frequency.
2. Description of the Background
There is a known auto-focus circuit that operates on the principle that the intermediate and high range levels in a luminance signal from a CCD imaging device are a maximum at the focus position. The intermediate and high range components in the luminance signal from the CCD imaging device are extracted by a high-pass filter and evaluated by integrating the level of the intermediate and high-range components within a predetermined focus area. The correct focus position is obtained by controlling the position of the lens to maximize the evaluated value. In a conventional auto-focus circuit of this kind, an analog, high-pass filter is used to extract intermediate and high-range components in a luminance signal from a CCD imaging device. This analog circuit, however, does not have good temperature characteristics, and is difficult to miniaturize. In this connection, there has been proposed a system for digitizing a video signal from the CCD imaging device and performing auto-focus control in a digital circuit.
In constructing the auto-focus circuit from a digital circuit, a digital high-pass filter is used to extract intermediate and high-range components in the luminance signal produced by from the CCD imaging device. The characteristics of the digital, high-pass filter vary with the sampling frequency of the digital signal. The CCD imaging devices typically employed are of two formats, for example, 760 pixels per line or 510 pixels per line. In the case of a CCD imaging device having 510 pixels per line, the sampling frequency is (8/3) of the color subcarrier frequency (fsc). In the case of the CCD imaging device having 760 pixels per line, the sampling frequency is 4 fsc.
Therefore, in order to construct an auto-focus circuit using digital, high-pass filters suitable for both the CCD imaging device having 510 pixels per line and the CCD imaging device having 760 pixels per line, it is necessary to convert the first sampling frequency of (8/3) fsc, or the second sampling frequency of 4 fsc into a common sampling frequency.
One approach would be to convert the digital signal of sampling frequency (8/3) fsc or 4 fsc from the CCD imaging device into a common frequency of 2 fsc. Nevertheless, if the digital signal of sampling frequency (8/3) fsc or 4 fsc is converted into the sampling frequency 2 fsc, a problem caused by aliasing will occur. Thus, if separate filters are provided for the digital signal of sampling frequency (8/3) fsc and for the digital signal of sampling frequency 4 fsc in order to remove the adverse effects of aliasing, the circuit will be large in scale and not subject to the miniaturization that is required of a video camera.